A single Mediator cannot handle multiple callbacks. If you have a composite application with a Mediator that receives multiple callbacks, the behavior of the composite application is undetermined. For example, in the scenario shown in Figure 20-9, AsyncMediator forwards the callback response from AsyncEchoMediator1 and AsyncEchoMediator2 to FileInMediator. In such a flow, the AsyncMediator might return the callback from both AsyncEchoMediator1 and AsyncEchoMediator2, or from either one of them. The exact behavior is random and unpredictable.